Silicon micromechanics may allow economical mass production of small sensor chips using semiconductor technology methods. Mass flow sensors having dimensions on the order of a few hundred micrometers are used, for example, in the automotive sector to control air delivery.
Mass flow sensors using silicon micromechanics from other systems, for example as discussed in German Published Patent Application No. 195 27 861, encompass a heating element and typically several temperature measurement elements, all of which are patterned out of a metal resistor layer and are positioned on a thin membrane made of dielectric material. The membrane spans a recess in a silicon substrate.
When a flow of medium, in particular an air flow, flows along the upper side of the membrane, that flow of medium causes cooling of the membrane. That cooling can be evaluated, in particular, using one temperature measurement element placed upstream (relative to the heating element) and a further temperature measurement element placed downstream, the measurement element placed upstream being more greatly cooled than the one placed downstream. Alternatively, the cooling of the membrane may be determined by measuring the resistance of the heating element. The temperature measurement elements are also resistors, which are made of a material whose resistance is temperature-dependent. One material that is suitable for the heaters and the temperature measurement elements is platinum.
German Published Patent Application No. 195 27 861 discusses the manufacture of a membrane sensor of this kind, to begin with a silicon substrate and firstly to deposit on its upper side a membrane layer made of silicon oxide, silicon nitride, or similar materials. A metal layer is then applied onto the entire surface of the membrane layer, and the measurement and heating elements, conductor paths, etc. are patterned out it by photolithography and etching. In a further process step the recess is etched in, proceeding from the back side of the silicon substrate, by manner of an etching frame, so that only a frame made of silicon, with the membrane spanned within it, remains. The underside of the membrane layer is then open (except where it covers the frame), i.e. it is bounded by air.
This membrane construction produces a thermal insulation, between the resistors patterned out of the metal layer and the substrate, that is ensured by the recess and the air present in it (the thermal conductivity of air or SiO2 is approximately two orders of magnitude less than that of silicon). Without a recess, a substantial portion of the heat generated by the heating element would not be dissipated by the medium flowing past, but rather would flow away through the membrane layer to the silicon substrate. This heat flowing away laterally and vertically into the substrate would be problematic simply because a predefined working temperature that is also important in terms of measurement sensitivity would be achievable, at best, only by increasing the energy consumption of the heater.
A lateral heat outflow would be problematic in terms of the measurement elements that are typically positioned laterally next to the heater, since their function is based on the existence of a temperature gradient. This gradient exists between the center of the membrane (heated by the heating element) and the edge of the sensor (which should be as close as possible to ambient temperature). With a lateral heat outflow, to which the direct thermal coupling via the membrane itself does not make a substantial contribution, the temperature measurement elements would also be heated via the substrate. The decreased temperature gradient between heater and measurement elements would result directly in decreased sensor sensitivity. Thermal decoupling in the lateral and vertical directions thus appears indispensable.
As discussed, however, the membrane sensors require for their manufacture a volume-based micromechanical process from the back side of the substrate. This patterning of the membrane may be accomplished via a KOH etching process in which the wafer is inserted into an etching box. The manufacturing method using bulk silicon micromechanics with an integrated back-side process is very complex and therefore associated with high cost. In addition, the thin membrane, with typical thicknesses of 1 to 2 um, has a tendency toward undesired vibrations due to its low mechanical stability.